The present inventors have proposed a secondary battery separator as a lithium secondary battery separator. The secondary battery separator includes a porous resin film which has a porosity of 60% or more, and has pores regularly arrayed in three-dimensions and being in mutual communication via through-holes (three-dimensionally ordered pores) (Patent Literature 1).
In a manufacturing method described in Patent Literature 1, monodisperse spherical silica particles having a particle size of 50 to 2500 nm are dispersed in a solvent. By filtering the dispersion liquid with a filter, the monodisperse spherical silica particles are accumulated on the filter, to form a close-packed structure. Spaces among the silica particles of a sintered body obtained by firing the deposit are filled with a resin. The sintered body is then immersed in a hydrofluoric acid solution, to dissolve and remove the silica particles, thereby forming through-holes. Since the monodisperse spherical silica particles have a uniform particle size, the monodisperse spherical silica particles are closely packed easily, and the through-holes formed after the silica particles are dissolved and removed also have the same size. However, the hydrofluoric acid solution is required in order to dissolve and remove the silica particles. The hydrofluoric acid solution is hard to handle, which disadvantageously causes a high manufacturing cost.
There has been also proposed a method of manufacturing a secondary battery separator comprising steps of forming a varnish from polyamic acid or polyimide, silica particles, and a solvent on a substrate, and imidizing the varnish to obtain a polyimide-silica composite film, and dissolving and removing silica using hydrogen fluoride water from the polyimide-silica composite film (Patent Literature 2). However, the method of Patent Literature 2 also uses the hydrofluoric acid solution, which causes a handling and cost problems as in Patent Literature 1.